1. Technical Field
The present invention relates to an electro-optical device such as a liquid crystal display device, and an electronic apparatus having the electro-optical device, such as a liquid crystal projector.
2. Related Art
According to the related art, in a liquid crystal display device employing an active matrix driving method using TFTs (thin film transistors), a plurality of scan lines and a plurality of data lines, which are arranged in horizontal and vertical directions, respectively, and a plurality of pixel electrodes formed at intersections of the scan lines and the data lines are provided on a TFT array substrate. In addition, on the TFT array substrate may be provided various peripheral circuits having TFTs as their elements, including a sampling circuit, a pre charge circuit, a scan line driving circuit, a data line driving circuit, and a test circuit, etc. If liquid crystal panels or liquid crystal modules including liquid crystal panels and peripheral circuits formed on the liquid crystal panels are equal in size, a larger image display region defined by the plurality of pixel electrodes arranged in the form of a matrix, that is, a larger region on the liquid crystal panel on which images are displayed depending on variation of alignment conditions of liquid crystal is more desirable as basically requested for a display. Accordingly, it is common that the peripheral circuits are provided in a narrow and long peripheral portion of the TFT array substrate around the image display region.
Of these peripheral circuits, the sampling circuit is a circuit for sampling an image signal having a high frequency so as to stably supply the image signal to the data lines at a predetermined timing in synchronization with scan signals. The sampling circuit requires sufficiently high current supply capability for TFTs as its main elements in order to exert its function as mentioned above. In addition, since current flowing through the TFTs composing the sampling circuit leaks out somewhat even in an off state under a specific voltage, channels of the TFTs must be long to some degree in order to suppress the leak current. Accordingly, the TFTs can not become small in size unconditionally. When there is a restriction on reduction of the channel length, channel widths of the TFTs have to be enlarged in order to realize the high current supply capability. Under such a constraint, conventionally, the sampling function is compatible with a layout in a narrow region by arranging sampling circuits around the image display region.
In addition, if the channel widths of the TFTs included in the sampling circuit increases, a distance at which image signal lines and data lines electrically connected to the TFTs increases. Accordingly, there arises a technical problem in that a size of capacitive coupling between these lines increase due to parasite capacitance between these lines and variation of potential on the image signal lines has an effect on potential on the data lines even when the TFTs of the sampling circuit are in an off state, consequently deteriorating image quality. More specifically, the enlargement of the channel widths of the TFTs may cause a so-called pushdown voltage effect that the potential of the data lines is turned to an image signal potential lower than an original image signal potential. As means to overcome such a technical problem, JP-A-2002-49357 and JP-A-2002-49331 disclose a technique for reducing parasite capacitance between the data lines and the image signal lines, which exists in the vicinity of switch circuits included in the sampling circuits.
However, in JP-A-2002-49357 and JP-A-2002-49331, each of the switch circuits is formed of one TFT, and, for example, a single channel TFT such as an n-channel TFT is in charge of maintenance of the image signal and record of the image signal on the data lines. With such a TFT, an amount of charges ejected from the TFT when the TFT is turned into an off state increases, thereby increasing pushdown voltages of the data lines. As a result, there arises a technical problem of deterioration of image quality due to a luminance difference between pixels electrically connected to the data lines. In addition, for example in an inverse-driven liquid crystal display device, record of the image signal at a positive electrode and record of the image signal at a negative electrode becomes asymmetrical due to the pushdown voltages, thereby causing an operational problem of malfunction such as burn-in of liquid crystal.